Electrostatic recording media are used in electrostatic imaging systems and comprise a support and a layer of dielectric material provided thereon. Electrostatic recording media are used to form images by first applying an electrical charge pattern on the dielectric material layer, for example by means of a stylus. The charge pattern is then rendered visible by the application of a toner or liquid developer comprising toner particles in a non-aqueous liquid. The image may be subsequently fixed to render it permanent by means known to those skilled in the art.
Various methods of manufacturing electrostatic recording media or dielectric papers are well known in the art. Typically, a non-aqueous solvent solution of a polymeric material may be coated onto the electrically conductive base paper. Alternatively, electrostatic recording media are prepared by extruding a polymeric material layer, i.e., the dielectric material layer, onto a support such as electrically conductive base paper.
The preferred method of manufacturing electrostatic recording media is to coat the dielectric material layer onto the electrically conductive paper from either an aqueous solution or an aqueous dispersion, for example a latex, containing a polymeric material. Although this method is environmentally safer, is less expensive and less complex than solvent systems, it still uses expensive electrically conductive paper.
The dielectric material layer can also be applied to a support or paper that is electrically non-conductive provided conductivity is produced in said support. Electrical conductivity of non-conductive paper has been achieved in the past by several different techniques. One technique employs inorganic salts which effectively penetrate paper which is porous. These materials function well under normal humidity conditions, however, they are not normally good conductors at relatively low humidities. A second technique involves the application of conductive polymers to both surfaces or sides of the paper by coating or size pressing. Electrically conductive papers prepared in this manner work well over a wide range of humidities and provided the dielectric coating is not water based. A barrier layer is also required between the aqueous dielectric coating and the electrically conductive paper to prevent contamination of the dielectric coating. The addition of a barrier layer results in an added cost to the final product and a loss of image density by virtue of reduced latent electrostatic field strength.
Another method of achieving electrical conductivity in non-conductive paper is to apply conductive clays to both sides of the non-conductive paper using a size press. Although this method provides papers having good conductivity, the approach is costly because conductive clay dispersions are expensive to make.
U.S. Pat. Nos. 4,739,003 and 4,868,048 issued to Barr et al. disclose an aqueous conductivizing composition for conductivizing paper and a conductive sheet material made from said composition. The aqueous conductive composition comprises a conductivizing agent and a binder. The conductivizing agent contains a synthetic hectorite clay which has been purified. Thus, an additional purification step and a binder is needed to make the conductive papers disclosed by Barr et al.
Accordingly, there is a need for an electrostatic recording media having good image performance, particularly at relatively low humidities, and which can be prepared simply and economically using preferably aqueous based coatings. These needs are met using the electrostatic recording media of the invention.